This invention is in the field of laser power control, and is more specifically directed to pulse-width modulated laser power control.
As is fundamental in the art, laser emission is achieved by the stimulated emission of photons from a laser medium, which may be a gas (e.g., CO.sub.2, Ar), solid material (e.g., Nd:YAG), liquid (e.g., dye lasers), or semiconductor (e.g., GaAlAs). Typically, electrons in the medium are pumped, or excited, to an upper energy level, above the ground state. The state at which sufficient electrons have been excited into the upper energy level to sustain the laser reaction is referred to in the art as population inversion. Some of the excited electrons spontaneously emit a photon, and drop to an energy level that is the excited state less the energy corresponding to the wavelength of the emitted photon. These emitted photons will, in turn, cause other electrons in the population inversion to similarly emit photons, resulting in the generation of coherent light (i.e., the well-known laser beam). Positive feedback techniques, such as parallel mirror optical feedback, permit the laser to oscillate, further increasing the beam energy.
As is evident from the foregoing description, and as is well known in the art, laser pumping requires significant energy to be absorbed by the laser medium before the laser reaction is initiated. In the case of pulse-width-modulated (PWM) lasers, the laser pump is either fully on or fully off at any instant, operating at a duty cycle according to the desired average laser power. However, energy of the laser medium will decay from the excited state toward the ground state over such time as the laser pump is off. After a significant off time (such as between laser pulses), the time required for the laser pump to again excite the medium into a population inversion may be significant, particularly relative to the duration of the first pulse in a PWM pulse stream. Indeed, at high PWM frequencies (i.e., short "on" pulses) and low duty cycles, the pulse width of the first "on" pulse may not be sufficiently long to reach the lasing threshold, in which case the laser may not emit energy at all during the first pulse. Accuracy in the generation of laser power during this first pulse after a significant time off is thus greatly compromised.
One conventional approach to addressing the first pulse problem is referred to in the art as a "tickle" scheme. According to this approach, which for example is used in CO.sub.2 lasers available from Synrad, Inc., the laser medium is constantly excited to a level just below the lasing threshold, for example by way of a low duty cycle pulse stream. In this condition, the time required to reach population inversion is kept very short, as the laser medium is energized (in theory) to a level that is only slightly below population inversion. Upon turning on the laser, the time required to raise the energy of the laser medium into population inversion is thus much reduced, such that the first pulse in the PWM stream is likely to generate the desired laser power. The "tickle" scheme has certain limitations, however, as its accuracy is quite dependent upon each particular laser; if the "tickle" duty cycle is too high for a particular laser, the laser may lase even in its "off" state. Because conventional lasers may degrade over time and use, a "tickle" duty cycle that is properly set for a laser at the beginning of its life may be inaccurate, and cause result in undesired lasing during "off" cycles, in later life.
Another conventional approach to addressing the first pulse problem is the implementation of a separate excitation amplifier, for example an RF excitation amplifier as used in CO.sub.2 lasers available from Coherent, Inc. The separate excitation amplifier is set to maintain the laser medium at an excited state slightly below population inversion, similarly as in the "tickle" process described above. The output of the excitation amplifier must also be precisely set for each laser and its expected operating conditions. If the excitation amplifier is set too high, undesired lasing during "off" periods may occur; conversely, if the excitation amplifier is set too conservatively, the turn-on characteristics of the laser may be less than desired, particularly at low PWM duty cycles.
Another known operation technique in the field of lasers is referred to as "Q-switching" (or, alternatively, "Q-spoiling"). Q-switching is commonly used to generate a high power pulse from a laser by switchably lowering the Q factor of the resonating laser, for example by inserting a damping element into the laser, allowing energy to build up in the laser medium without emission of significant light. At such time as the pulse is desired, generally after the medium is excited well beyond the population inversion level, the Q factor is again raised (for example, by removing a damping element in the laser), releasing a large pulse of laser energy. Q-switching is commonly used in continually-pumped, solid-state lasers. In the example of solid-state laser, the damping element is placed in the optical path to prevent oscillation until the time that the laser pulse is desired. The turn-on time of Q-switched lasers is quite fast, given that the medium is maintained at an energy level above population inversion prior to switching. However, as is well known in the art, the Q-switch adds significant cost into the laser, especially considering the cost of the absorber (e.g., Pockels cell).
The sensitivity to inaccuracy of the laser power in a first pulse is exacerbated in some laser applications. For example, one use of lasers in the art is the fabrication of three-dimensional articles in layer-wise fashion through the selective polymerization of photopolymer liquid (as in the well-known stereolithography process), through the selective fusing, melting, or sintering of powder (as in the well-known selective laser sintering process), or through laminated object manufacturing (LOM). In these types of manufacturing methods, laser power is a critical parameter in the fabrication of structurally sound articles.
By way of background, an example of a rapid prototyping technology is the selective laser sintering process practiced in systems available from DTM Corporation of Austin, Tex., in which articles are produced from a laser-fusible powder in layerwise fashion. According to this process, a thin layer of powder is dispensed and then fused, melted, or sintered, by laser energy that is directed to those portions of the powder corresponding to a cross-section of the article. Conventional selective laser sintering systems, such as the SINTERSTATION 2500 system available from DTM Corporation, position the laser beam by way of galvanometer-driven mirrors that deflect the laser beam. The deflection of the laser beam is controlled, in combination with modulation of the laser itself, to direct laser energy to those locations of the fusible powder layer corresponding to the cross-section of the article to be formed in that layer. The laser may be scanned across the powder in raster fashion, with modulation of the laser effected in combination therewith, or the laser may be directed in vector fashion. In some applications, cross-sections of articles are formed in a powder layer by fusing powder along the outline of the cross-section in vector fashion either before or after a raster scan that "fills" the area within the vector-drawn outline. In any case, after the selective fusing of powder in a given layer, an additional layer of powder is then dispensed, and the process repeated, with fused portions of later layers fusing to fused portions of previous layers (as appropriate for the article), until the article is complete.
Detailed description of the selective laser sintering technology may be found in U.S. Pat. No. 4,863,538, U.S. Pat. No. 5,017,753, U.S. Pat. No. 5,076,869, and U.S. Pat. No. 4,944,817, all assigned to Board of Regents, The University of Texas System, and in U.S. Pat. No. 4,247,508 assigned to DTM Corporation, all incorporated herein by this reference. Description of a laser power control system in such a selective laser sintering system, in which the power of the laser is controlled according to the speed of the laser scan to provide a constant laser power output, is provided in copending U.S. application Ser. No. 08/866,600, filed May 30, 1997, assigned to DTM Corporation, and also incorporated by reference hereinto. The selective laser sintering technology has enabled the direct manufacture of three-dimensional articles of high resolution and dimensional accuracy from a variety of materials including wax, polycarbonate, nylon, other plastics, and composite materials such as polymer coated metals and ceramics. Wax parts may be used in the generation of tooling by way of the well-known "lost wax" process.
The thermal fusing mechanism in the selective laser sintering process depends upon the laser energy flux density, which is the thermal energy received to each location of powder per unit time. The laser flux density depends upon the laser energy, the laser spot size, and the temporal duration of the exposure of the powder to the laser energy. For most materials, the shape and density of the article being formed is sensitive to the laser energy flux density, such that even slight deviations in the laser energy flux density result in less than ideal article attributes. For example, if the laser energy flux density is too low, the article will be mechanically weak; conversely, excessive laser energy flux density can result in poor fidelity of the article dimensions relative to the CAD representation, overheating of the powder, or even burning of the powder.
The effects of non-uniformity of laser energy flux density have been observed in articles fabricated by selective laser sintering, especially in the sensitive materials noted above. It is contemplated that excessively low laser power in the initial laser pulse of a scan of the fusible material, such as may result from the delay in excitation of the laser medium to population inversion, will cause incomplete formation at the edges of the article, resulting in dimensional distortion of the object or reduced structural strength. Such sensitivity to laser power in the initial pulse is contemplated to not only affect the quality of articles formed by selective laser sintering technology, but also in other laser-based technologies such as stereolithography in which inadequate photocuring at article edges may result from loss of laser power in initial pulses.